1. Field of the Invention
The present invention relates to a liquid electrophotographic image forming apparatus, and more particularly, to a liquid electrophotographic image forming apparatus having a unit for depositing a predetermined amount of a liquid carrier on a sheet of paper before an image is transferred onto the sheet of paper.
2. Description of the Related Art
Typically, a liquid electrophotographic printer forms an electrostatic latent image on a photosensitive medium, such as a photosensitive belt or a photosensitive drum, develops the electrostatic latent image with a developing agent having a predetermined color, and transfers the developed electrostatic latent image onto a recording sheet, thereby obtaining a desired image. The electrophotographic printer is largely categorized into liquid and dry electrophotographic printers depending on the type of developing agent. The liquid electrophotographic printer uses a liquid developing agent, which is a mixture of a powder toner and a volatile liquid carrier.
Since the liquid electrophotographic printer using a liquid developing agent uses toner having a fine particle size of approximately 0.5–5 μm, the liquid electrophotographic printer obtains a high-quality image compared to the dry electrophotographic printer using a powder toner when an electrostatic latent image is developed, and harmfulness caused by toner dust is prevented.
FIG. 1 shows a conventional liquid color image forming apparatus disclosed in U.S. Patent Publication Application No. 2003/0044202. Referring to FIG. 1, a plurality of image forming units are arranged in a series on a circulation path of a transfer belt 40 which travels in a closed endless loop by a plurality of rollers 41, 42, and 44. Each image forming unit transfers one of four color images, such as a yellow (Y) image, a cyan (C) image, a magenta (M) image, and a black (K) image, onto the transfer belt 40.
Each image forming unit includes a photosensitive drum 30 which transfers a toner image having a predetermined color onto the transfer belt 40, a charging roller 36 which charges the surface of the photosensitive drum 30 to a predetermined potential, a laser scanning unit (LSU) 38 which forms an electrostatic latent image by radiating light onto the surface of the photosensitive drum 30, and a developing unit which develops the electrostatic latent image to a predetermined toner image. An eraser 34 erases charge on the photosensitive drum 30.
The transfer belt 40 is supported by a driving roller 41, a secondary transfer backup roller 44, and a plurality of primary transfer backup rollers 42 which correspond to the photosensitive drum 30, and rotate, and an image formed on the photosensitive drum 30 is transferred onto the transfer belt 40. A secondary transfer roller 43 faces the secondary transfer backup roller 44 having the transfer belt 40 placed therebetween. The secondary transfer roller 43 transfers the image on the transfer belt 40 onto a sheet of paper 60.
A sheet of paper onto which a color image is transferred by the secondary transfer roller 43, is heated at a predetermined temperature and pressed by a fusing unit 50, which is installed on a paper exhaust path so that the toner image is fused on the sheet of paper 60.
The developing unit includes a developing container 14 in which a developing agent is stored, a developing roller 15, a part of which being soaked in the developing container 14 to face the photosensitive drum 30, a deposit roller 16, a cleaning roller 17, and a metering roller 18.
The developing roller 15 is partly soaked with the developing agent stored in the developing container 14. The deposit roller 16 is installed and soaked with the developing agent stored in the developing container 14. A predetermined voltage is applied to the deposit roller 16 when the deposit roller 16 is spaced a predetermined gap from the developing roller 15. The deposit roller 16 attaches the developing agent to the surface of the developing roller 15 using a potential difference between the deposit roller 16 and the developing roller 15 having a predetermined voltage.
The metering roller 18 is installed adjacent to the developing roller 15 and regulates the developing agent attached to the developing roller 15 by a predetermined thickness.
A cartridge 19 accommodates toner having a predetermined color with a high concentration to be supplied to the developing container 14.
Meanwhile, the developing agent stored in the developing container 14 is mixed with a norpar carrier so that a solid concentration of toner is approximately 5–20 percent by weight. A toner layer having a solid concentration of approximately 25–30 percent by weight is formed on the developing roller 15 by mass/area (M/A) 100–300 μg/cm2. The toner layer develops the photosensitive drum 30, and a toner image is developed on the photosensitive drum 30, a color yellow image is transferred onto the transfer belt 40.
By performing development and transfer operations in the order of the image forming units, each color image is superimposed on and transferred onto the transfer belt 40 so that one color image is formed on the transfer belt 40. The transfer belt 40 is formed of a semiconductive material and does not absorb the carrier.
Subsequently, the color image on the transfer belt 40 is transferred onto a sheet of paper 60 that is fed between the secondary transfer roller 44 and the secondary transfer backup roller 43. Thus, when toner has a positive charge, an electrostatic force is applied to the toner so that a negative voltage is applied to the secondary transfer roller 44 and the color image on the transfer belt 40 is transferred onto the sheet of paper 60.
The amount of charge of dry toner is approximately 50 μC/g, whereas the charge amount of liquid toner is 2–10 times larger than the charge amount of the dry toner. Thus, when the liquid toner having a charge amount larger than the dry toner is transferred onto a sheet of paper, a high electrostatic force is applied to the sheet of paper. However, when a dielectric constant of the sheet of paper is low, transfer efficiency is reduced. In addition, because the dielectric constant of the sheet of paper varies according to the type of paper, printing quality also varies.